The Annual Variation and Its Influencing Mechanism of Surface Roughness Length of Yuzhong in Semi-arid Areas

Based on data observed at the Semi-arid Climate and Environment Observatory of Lanzhou University (SACOL) from June 2006 to December 2010, temporal characteristics of aerodynamic roughness length over the natural vegetation surface of Yuzhong are analyzed. Annual change characteristics of roughness length and influencing mechanisms in the southeast and northwest are analyzed, taking the impact of terrain, vegetation, precipitation and thermal conditions into account, and the fitting relationships between normalized roughness and time are given. It shows that for heterogeneous underlying surface, the difference of aerodynamic roughness length in different wind directions caused by undulating terrain and vegetation difference is very significant. According to the prevailing wind direction, two wind directions which are southeast and northwest are selected. Both magnitude and changing trends of roughness length of two selected wind directions are remarkable different. The averaged roughness length in southeast is 0.015 m, whose magnitude is equal to the roughness length over sparsely vegetated underlying surface like deteriorated grassland, while the averaged roughness length in northwest is 0.123 m, whose magnitude is equal to the roughness length over farmland underlying surface. To eliminate effects of the inter-annual variation of roughness length, the normalized roughness length is injected into the discussion. The time-distributing characteristics in two wind directions vary considerably, which can be considered showing opposite trends. The annual changing trend of roughness length in southeast decreases first and then increases, while it increases first and then decreases in northwest. And due to differences in terrain and vegetation, influencing mechanisms of the time variation of roughness length in the two wind directions are different. The annual variation trend of normalized roughness is consistent with the annual variation of atmospheric stability and the roughness length has a certain relationship with atmospheric stability in southeast due to the stunted sparse vegetation. While the annual variation trend of roughness length is consistent with the annual variation of precipitation and the roughness length has a good relationship with precipitation in northwest due to the impact of vegetation, and the vegetation is mainly effected by the precipitation. The time parametric relationship between normalized roughness and time in two directions can be described by a set of sinusoidal functions, and the related coefficient can reach 0.49 and 0.82, respectively.